1. Field of the Invention
The present invention is directed to an automatic conveyorized solder mask vacuum applicator and method of operation thereof having utility in the application of dry films which conform to the contours of irregular surfaces such as the surfaces of printed circuit boards or other substrates having raised, typically electrically conductive, traces thereon. The applicator and method have particular utility for conveying and for applying heat and vacuum and mechanical pressure to printed circuit boards or substrates that prior to such application have had dry film loosely applied to at least one of the surfaces thereof as discrete cut sheets within the confines of the printed circuit board.
2. Description of the Prior Art
A solder mask is a hard, permanent layer of non-conductive material which covers the surface of a printed circuit board or other substrate, encapsulating the traces of the printed circuit itself. By solder mask is meant herein a hard, permanent layer which meets at least the minimal requirements of the abrasion resistance tests as defined in IPC-SM-840A, Table 12, Summary of Criteria for Qualification/Conformance (Institute for Interconnecting and Packaging Electronic Circuits). The solder mask is patterned to fully cover the circuitry, except for those portions intended to be exposed, e.g., for soldering to another component. Solder masks are typically formed from a layer of photoimageable composition which is applied to a surface of the printed circuit board. The photoimageable layer is exposed to actinic radiation which is patterned by means of a template or artwork. Subsequent to exposure, the photoimageable layer is developed in an organic solvent or an aqueous solution which washes away either exposed or unexposed portions of the layer (depending upon whether the photoimageable material is positive acting or negative acting). The portion of the layer which remains on the surface is then cured, e.g., with heat and/or UV light, to form a hard, permanent solder mask intended to protect the printed circuitry for the life of the board.
One prior art method of applying the layer of photoimageable composition to the circuit board surface is to apply the material in liquid form, and then, either allow it to dry or partially cure the material to form a semi-stable layer.
There are a number of advantages to applying a photoimageable layer to a circuit board as a dry film rather than as a liquid. In particular, dry films are free of organic solvent and therefore eliminate this hazard from the workplace and eliminate the need for apparatus to protect the immediate work environment and the more general environment from organic solvent emissions. Typically, a dry film comprises a cover sheet of support material which is somewhat flexible but which has sufficient rigidity to provide structure to a layer of photoimageable composition which overlies one surface of the cover sheet. The cover sheet may be formed of polyester material, such a polyethylene terephthalate (PET), such as that sold as MELINEX.RTM..
To protect the photoimageable layer and to enable the dry film to be rolled, it is conventional for the exposed surface of the photoimageable layer to be covered with a removable protective sheet, e.g., a sheet of polyethylene. An example of such a dry film is sold as LAMINAR DM.RTM. by Morton International, Inc.
The method of use of such prior art dry film is generally as follows. The protective sheet is removed from the photoimageable composition layer immediately prior to application of the dry film to the surface of the printed circuit board. This may be accomplished, for example, using automated apparatus which peels away and rolls up the protective sheet as the dry film is unrolled from a reel. The dry film is applied to the surface of the circuit board with the photoimageable layer in direct contact with the board surface. Using heat, vacuum and mechanical pressure, the photoimageable layer is immediately laminated to the surface of the board. The cover sheet remains overlying the photoimageable layer, protecting the photoimageable layer from exposure to oxygen and from handling damage. The cover sheet also permits a pattern (or template) to be laid directly on top of the dry film for contact printing, if contact printing is to be used (as is usually preferred from the standpoint of obtaining optimal image resolution). The dry film is exposed to patterned actinic radiation through the PET cover sheet. At this time, the PET cover sheet is removed, permitting access to the exposed photoimageable layer by developer. Depending upon the composition of the photoimageable layer, the photoimageable layer is developed with organic solvent, aqueous developer, or semi-aqueous developer. By semi-aqueous developer is meant herein a developer which is about 90% or more by volume aqueous solution with the balance being an organic solvent such as 2-butoxy ethanol and other glycol ethers. The photoimageable layer may either be positive acting, in which case the exposed portions are removed by developer, or negative acting, in which case the unexposed portions are removed by developer. Most photoimageable layers for preparing solder masks are negative acting. Most photoimageable composition layers require some cure subsequent to development to render the layer hard and permanent so as to serve as a solder mask. Depending upon the composition of the photoimageable layer, curing may be effected with heat and/or UV light.
Printed circuit boards almost invariably have uneven surfaces in which circuitry traces are raised or elevated over the surface of a board of electrically non-conducting material. Circuitry traces may be the residual portions of an etched metal layer or may be built up from the board surface. It is desirable that a solder mask, particularly one formed from a photoimageable composition, conform to the contours of a circuit board surface. A conforming solder mask which adequately covers both the board surface and the upstanding traces minimizes the use of expensive photoimageable composition.
Processes for applying conforming solder mask on a surface having raised areas such as circuit traces on a printed circuit board are disclosed in U.S. Pat. Nos. 4,889,790 Leo Roos et al. and 4,992,354 F. J. Axon et al. and in for U.S. patent application Ser. No. 480,487 filed Feb. 14, 1990. These patents and application are assigned to the assignee of the present invention. The disclosures thereof, by reference, are incorporated herein.
The processes disclosed in these patents and application involve applying a solder mask-forming photoimageable composition layer to a printed circuit board using a dry film in which an intermediate layer is interposed between a support film or cover sheet and the photoimageable layer. The intermediate layer of the dry film is selectively more adherent to the photoimageable composition layer than to the cover sheet, allowing the cover sheet to be removed after the photoimageable layer is applied to a printed circuit board with the intermediate layer remaining on the photoimageable composition layer as a "top coat." The top coat is of non-tacky material and can be placed in contact with other surfaces, such as artwork for contact printing. The top coat also serves as an oxygen barrier, allowing the photoimageable composition layer to remain unexposed on the printed circuit board, after cover sheet removal, for some length of time. The use of dry film having the "intermediate layer" or "top coat" make possible the processes described in these patents and application. In each case there is provided a conforming step, e.g., conforming vacuum lamination, after removal of the cover sheet. Because the cover sheet is removed prior to the conforming step, better conformance, particularly when applying thin photoimageable composition layers onto boards with closely spaced traces, is achieved. Better resolution is also achievable because the top coat may be directly contacted with artwork for contact printing and because the top coat is much thinner than a cover sheet or support film and is, therefore, much less a deterrent to good resolution than a support film.
To form a solder mask, the protective, removable sheet of the dry film is first peeled away and the exposed surface of the photoimageable composition layer is applied to the surface of the printed circuit board. Using heat, vacuum and mechanical pressure, the dry film is laminated to the surface of the printed circuit board, partially conforming the photoimageable layer thereto. Within about 60 seconds and before substantial cooling of the printed circuit board and dry film has occurred, the cover sheet of the dry film is removed, whereupon the photoimageable composition layer and overlying top coat fully conform to the contours of the printed circuit board and substantially encapsulate the traces. The photoimageable composition layer is then exposed to patterned actinic radiation through the top coat. A developer is used to remove either exposed or non-exposed portions of the photoimageable composition layer, leaving the remaining portion of the layer laminated to the circuit board. Subsequently, the portions of the photoimageable composition layer remaining on the circuit board are cured, e.g., with heat and/or UV light.
In U.S. Pat. No. 4,946,524 granted on Aug. 7, 1990 to Robert C. Stumpf et al., the disclosure of which patent, by reference, is incorporated herein, there is disclosed an applicator and process for applying dry film solder mask material to the surface of a printed circuit board allowing, at the same time, handling of the board with the applied film, the draw-off of the air enclosed between the film and the board, and the removal of the cover sheet. The draw-off of air enclosed between the dry film and the surface of the printed circuit board is facilitated when, before vacuum lamination, the surface of the board is covered with a loose sheet of film. To that end the applicator of U.S. Pat. No. 4,946,524 is operative to tack the dry film to a board at the leading and trailing edges with the intermediate portion of the film loosely applied thereto. The film is tacked to the board as a discrete cut sheet within the confines of the perimeter of the surface of the board. For convenience, a printed circuit board having such loose application of a dry film sheet to the surface or surfaces thereof is referred to hereinafter as being "prelaminated."
The results of the processes described above have been most encouraging. Difficulty has been encountered, however, in attempting to adapt these processes for continuous automatic operation in an in-line system. This is particularly true with respect to the utilization of existing vacuum laminating apparatus in an in-line process. The present invention was devised to fill the gap that has existed in the art in this respect.